RAPID SLOPE EVOLUTION FOLLOWING THE 70,000 M3 2003 ASTILLA SLIDE IN MONTEREY CANYON

Time serial multibeam bathymetry data with high spatial precision are used to evaluate the geomorphic evolution of the Monterey Canyon head. Nine bathymetric surveys conducted between September 2002 and Fall 2006 document sand wave migration, net excavation of the canyon axis, gully head extensions, and a submarine landslide. During the 29 months between Fall 2002 and Winter 2005, the canyon grew in volume 1,000,000 m3 ± 700,000 m3, at an average annual rate of 400,000 m3/a ± 300,000 m3/a. The most significant processes were lateral erosion and vertical incision of the canyon axis. This net loss of substrate during the 29-month period is divided between local erosion of 1,400,000 m3 and local deposition of 350,000 m3.

A new submarine landslide was found on a 30 degree canyon wall at a depth of 130 m by comparing bathymetric surveys separated by only six months (March 2003 and September 2003). In September 2003 the slide scar was 50 m wide, 165 m long, and 8 m deep (average slope-perpendicular thickness). The scar had a 70,000 m^3 volume, and the debris pile had a volume of only 52,000 m^3, suggesting that 18,000 m^3 debris was transported down-canyon following the slide. By February 2005 the scar had grown approximately 30% to 100,000 m^3 by increases in width, length and depth. During the same time frame, the debris pile diminished to 11,000 m^3, leaving 88,000 m^3 of debris removed from the toe of the slide. Paleo-tsunami reconstructions based upon present day scar volumes may be in error if significant enlargement is common in submarine landslide scars.

Photos from an ROV reconnaissance in Fall 2006 indicate that the canyon wall in the slide scar is well stratified. Shallow cores show that the slide material is mainly poorly consolidated mudstone. The ROV data indicate that the toe of the canyon wall near the toe of the slide is cleanly excavated and undercut. Apparently erosion of the toe of the slope is the mechanism for slide generation in this part of the canyon.